During audio system start-up and shutdown, disturbances originating in various parts of the system result in an irritating sound or "pop" emanating from the speakers. A number of proposals for solving this problem have been made in the prior art. It has been proposed to delay the turn-on of the power amplifiers so that such transient disturbancees do not reach the speakers during system start-up. See for example Figure 4.13.27 of the 1980 Audio/Radio Handbook from National Semiconductor Corporation. Ozawa et al U.S. Pat. No. 4,127,743 proposes a relay control switch between the power amplifier and the speaker and control circuitry for delaying closure of this switch when power is applied and to very quickly open the switch when power is removed. Kawada U.S. Pat. No. 3,840,820 provides a muting transistor which shunts the input to the power amplifier to ground for a short time interval when power is applied and removed from the system. Morse U.S. Pat. No. 3,916,321 also employs a transistor for muting the audio signal channel during power-up and power-down of the system.
With the advent of higher performance audio systems for the automobile, there are various advantages to mounting the power amplifiers in the remotely located speaker enclosures. Such power amplifiers usually include muting circuitry which senses the supply voltage in order to mute the power amplifier during start-up and shutdown. While the muting circuitry is effective during start-up, the filter circuitry in the power amplifier prevents the muting circuitry from performing its function for a predetermined time interval after removal of power. This has the salutary effect of preventing inadvertent muting during power supply variations, which are prevalent in automobiles, but does permit noise generated during shutdown of the audio processing circuitry to be heard. In order to eliminate this time interval and activate the mute circuitry immediately upon removal of power, a separate mute wire is required and where several speakers are employed, this results in a cost penalty and a potential reliability problem during vehicle assembly and product service life.